The instant invention, as disclosed and described herein, relates to apparatus for continuously depositing layers of semiconductor alloy material onto a substrate in each of at least one deposition chamber through which the substrate is adapted to successively travel. In the preferred embodiment, the deposition of amorphous semiconductor alloy material is accomplished by glow discharge deposition techniques in which a gaseous precursor material is decomposed and recombined into molecular species and compounds under the influence of an electromagnetic field developed in the plasma region of the deposition chamber. The composition of the amorphous semiconductor alloy material deposited onto the substrate is dependant upon, not only (1) the composition of the particular gaseous precursor material which is introduced into the deposition chamber, but also (2) the particular molecular structure of the molecular species and compounds at the time of introduction into the plasma region, (3) the manner in which those molecular species and compounds react when subjected to the electromagnetic field, (4) the rate at which those molecular species and compounds are depleted during the decomposition and deposition process, and (5) the length of time those molecular species and compounds are subjected to the electromagnetic field prior to deposition onto the substrate. It should therefore not be surprising that it is important to carefully monitor and control, not only the relative quantities and compositions of the various gaseous precursor materials introduced into the plasma region of the deposition chamber, but also (1) the molecular structure of the semiconductor alloy material deposited across the entire surface area of the substrate and (2) the length of time the various molecular species and compounds are subjected to the electromagnetic field prior to the depletion thereof. To the end of controlling the composition of the gaseous precursor materials introduced into the plasma region, the deposition chamber is sealed from atmospheric conditions, pumped to a relatively low pressure, heated to high temperatures, and flushed with gaseous hydrogen, argon or nitrogen prior to initiation of the glow discharge process. The deposition chamber may further be preferably, but optionally, equiped with a precathode assembly for (1) removing contaminants from walls of the chamber and impurities from the gaseous precursor material and initiating the disassociation and (2) recombination of the gaseous precursor materials prior to the time that those materials flow into the plasma region. The concept of a precathode assembly has been fully disclosed in the U.S. patent application Ser. No. 452,224 filed Dec. 12, 1982, entitled "Upstream Cathode Assembly" assigned to the Assignee of the instant patent application and the subject matter of which is incorporated herein by reference. Said precathode assembly, fully described therein, substantially solved the problem of uniformity of the layers of semiconductor alloy material which were deposited in both static and batch-type glow discharge deposition systems. Further, U.S. Pat. No. 4,479,455 filed Mar. 14, 1983, entitled "Reaction Gas Introduction and Channeling System", also assigned to the assignee of the instant patent application, and the subject matter of which is also incorporated by reference, dealt with and substantially solved the problem of providing uniformity (across the entire surface of a large area substrate) of the deposited semiconductor alloy layers in deposition systems employing a continuously moving substrate.
In order to more fully understand the problems associated with the deposition of profiled semiconductor alloy layers which must exhibit uniform optical, electrical and chemical characteristics over large areas (at least approximately one square foot), it is necessary to describe, in greater detail, sophisticated process gas introduction systems, such as the system illustrated and described in U.S. patent application Ser. No. 475,289, mentioned hereinabove. Note that when discussing the uniform deposition of semiconductor alloy material, it is common to refer to the development of an electromagnetic field by the transmission of electromagnetic energy to a cathode or antenna, the field being formed between the deposition cathode or antenna and a grounded substrate spaced therefrom. The area in which this electromagnetic field acts will hereinafter be referred to generally and interchangeably as either the "decomposition region" or the "plasma region". It is upon entering this plasma region and being subjected to the electromagnetic field that the gaseous precursor materials are disassociated and recombined into an ionized plasma made up of molecular species and compounds of the originally introduced gases. The ionized gaseous species and compounds are deposited onto the heated surface of said substrate as it passes through the plasma region of the deposition chamber. In order to avoid the problem which was previously encountered when using glow discharge processes to deposit semiconductor alloy material onto a moving substrate, even when the deposition apparatus was equipped with the precathode assemblies, i.e., the problem of having semiconductor alloy material deposited adjacent the upstream section of the cathode (that section of the substrate first contacted by process gases introduced into the plasma region) exhibit different (non-uniform) optical, electrical and chemical characteristics than the semiconductor alloy material deposited at more downstream sections of the cathode (those sections of the cathode later contacted by the process gases as they pass through the plasma region), the gaseous precursor materials were introduced and made to flow through the plasma region in a direction which is substantially parallel to the direction of travel of the web of substrate material through the plasma region of the deposition chamber. Further note, that while the direction of flow of the gaseous precursor material through the plasma region is described as parallel to the direction of movement of the substrate therethrough, that direction of flow may be either in the same direction of movement or in an opposite direction of movement relative to the web of substrate material.
Regardless of the direction of flow, i.e., as long as the gaseous precursor materials are made to flow in a direction substantially parallel to the direction of travel of the web of substrate material, the semiconductor alloy material deposited thereonto, although graded (cross-sectional slices of the material taken in a direction parallel to the plane of the web of substrate material as it travels through the deposition chamber may vary in homogeniety with respect to one another), is substantially "uniform" (cross-sectional slices of the material taken in a direction perpendicular to the plane of the web of substrate material, which slices comprise an aggregation of the graded layers, exhibit substantially similar chemical, optical and electrical properties). This uniformity was achieved because the gaseous precursor materials present at any arbitrarily selected line extending across the entire transverse width of the web of substrate material are substantially identical and therefore the gaseous precursor materials which are present adjacent any arbitrarily selected line extending across the entire transverse width of the web of substrate material are subjected to the effects of the electromagnetic field established between the cathode and the web of substrate material for substantially identical lengths of time. The molecular species and compounds formed from those disassociated and recombined gaseous precursor materials, being at substantially identical stages of disassociation and recombination, are therefore deposited onto the entire surface of the web of substrate material as a film of substantially uniform semiconductor alloy material, which material exhibits said substantially uniform optical, electrical and chemical properties.
Referring now specifically to FIGS. 3A, 3B, 4A, and 4B, the shortcomings of aforementioned U.S. patent application Ser. No. 475,289 with respect to the deposition of profiled layers of semiconductor alloy material will be described. As illustrated in FIG. 3A, the cathode assembly of a glow discharge deposition chamber, such that of the '289 application, is depicted generally by the reference numeral 50. In the cathode assembly 50, the web of substrate material 11 is shown moving in the direction of arrow A across, but spaced from, the cathode plate 34 as the gaseous precursor semiconductor material is introduced to flow from the gas inlet manifold 36 operatively disposed beneath the cathode plate 34, around a first edge of that cathode plate 34, through the plasma region developed between the upper surface of the cathode plate 34 and the moving web of substrate material 11, about the distal edge of the cathode plate 34 and through an exhaust 41 operatively disposed therebeneath. As previously mentioned, by introducing the gaseous precursor semiconductor material into the plasma region of the deposition chamber in a direction substantially parallel to the plane of travel of the moving web of substrate material 11, the deposited semiconductor alloy material, if measured by cutting cross-sectional slices in a direction transverse to the plane of the web of substrate material, would exhibit substantially uniform properties across the entire length and width dimensions thereof. Further, the semiconductor alloy material deposited onto the web of substrate material 11 in any given deposition chamber is an aggregation of the semiconductor alloy material deposited on an infinite number of hypothetical lines, each line extending across the width, and substantially parallel to the plane, of the web of substrate material 11. It is this aggregation of layers, each layer of which (being of infinitely small cross-sectional thickness) is of substantially uniform and homogeneous chemical composition, while adjacent layers differ from one another in uniformity and chemical composition. In summary then, while the deposited semiconductor alloy material might be non-uniform and vary in optical, electrical and/or chemical characteristics if, as above, a plurality of cross-sectional slices of that deposited material were taken in a direction parallel to the plane of the web of substrate material 11 and those slices were compared, the deposited semiconductor alloy material would be substantially uniform, graded and exhibit similar optical, electrical and chemical characteristics if a plurality of cross-sectional slices were taken in a direction perpendicular to the plane of the web of substrate material 11 and then compared.
The shortcomings of the construction of the cathode assembly 50 of FIG. 3A, previously alluded to, may now be explained with reference to the elongated, transversely extending, profiling gas manifold 52 positioned at the downstream end of the cathode plate 34. The profiling gas manifold 52 preferably includes a plurality of spaced apertures for introducing the profiling gases across the entire width of the decomposition region in a direction, depicted by the reference arrow D, said direction of flow of the profiling gases being substantially parallel to the direction of movement of the web of substrate material 11 through the decomposition region of the glow discharge deposition chamber, but opposite to the direction of flow of the gaseous precursor material through that decomposition region. The utilization of this type of profiling gas introduction system was believed to be diffusion limited by the flow of the relatively small amounts of profiling gas in a direction opposite to the direction of flow of the relatively high amounts of primary gaseous precursor semiconductor material through the decomposition region. In this manner, the flow of the primary gaseous precursor semiconductor material was used to restrict the levels of profiling gas reaching and traveling through downstream regions of the decomposition region. The concentration of profiling gas present in localized areas of the decomposition region would thereby vary in the direction D of the flow of the profiling gas. More particularly, the greatest concentration (albeit, a very low concentration) of profiling gas is located adjacent the downstream (relative to the direction of travel C of the gaseous precursor material) section of the decomposition region. Due to ( 1) the deposition of some of the profiling gas as that gas is subjected to the effects of the electromagnetic field developed in the decomposition region, and (2) the difficulty the profiling gas experiences in moving against the flow of the gaseous precursor material in reaching upstream locations within the decomposition region, the concentration of the profiling gases decreases with the upstream movement thereof through the decomposition region. In this manner, it was thought that by introducing, for instance, a p-dopant gas through the profiling manifold 52, the greatest amount of p-dopant gas would be deposited directly atop the p-doped semiconductor alloy layer just entering the, for instance, intrinsic deposition chamber 30. As the web of substrate material 11 moves through the decomposition region of that hypothetically intrinsic chamber 30, lesser amounts of p-dopant profiling gas are present and therefore deposited onto the web of substrate material. In this manner, the profiling of the intrinsic semiconductor alloy layer with successively lesser amounts of p-doped material from the p-dopant/intrinsic junction to the intrinsic/n-dopant junction is accomplished.
Referring to FIG. 3B, the profile of, for instance, the deposited p-doped semiconductor alloy material is illustrated by the solid line indicated by the reference numeral 56, while the dashed line 58 refers to semiconductor alloy material introduced into the downstream portion (relative to the direction of travel of the web of substrate material 11) of the decomposition region of the slow discharge deposition chamber through an elongated gaseous precursor material introduction manifold 54 (shown in phantom outline in FIG. 3A). More particularly, the second introduction manifold 54, is intended to introduce a gaseous precursor material into the plasma region, which material, when subjected to the effects of the electromagnetic field established between the cathode and the web of substrate material 11, is decomposed and deposited, in a graded manner, onto that web of substrate material 11 more quickly than the gaseous precursor semiconductor material introduced into the decomposition region from the main gas introduction manifold 36. Because of the more rapid deposition of the additional gaseous precursor material introduced through introductory manifold 54, the gaseous species introduced therefrom are continuously being depleted before reaching the downstream portions of the decomposition region, and hence are graded into the host matrix of the semiconductor alloy material which is simultaneously deposited onto the web of substrate material 11.
Turning now to FIGS. 4A and 4B, the cathode region of a further embodiment of a glow discharge deposition chamber disclosed in U.S. Pat. No. '455 is depicted generally by the reference numeral 60, said cathode region representing a mirror image of the gas introduction and channeling system described with reference to FIGS. 3A and 3B. This mirror-image gas introduction and channeling system can be employed to provide for the profiling of n-dopant gases for grading, the intrinsic/n-dopant junction of a p-i-n type photovoltaic device. More particularly, in FIG. 4A the web of substrate material 11 is again moving through the decomposition region in a direction of travel depicted by arrow A, however, the gaseous precursor semiconductor alloy material introduction manifold 36' is now operatively disposed so as to direct the flow of the semiconductor alloy gases through the decomposition region in the direction of arrow C', that direction being substantially parallel to and in the same direction as the direction of travel of the web of substrate material 11, vis-a-vis, the opposite direction in which the semiconductor alloy gases were made to flow through the decomposition region of FIG. 3A. After passing through the decomposition region, the semiconductor alloy gases are removed through an exhaust 41 disposed beneath the cathode plate 34. Grading of the dopant material in the semiconductor alloy host matrix is achieved through the use of a profiling gas introductory manifold 52' which is again adapted to introduce gaseous profiling material to flow through the decomposition region in the direction of arrow D', which direction of flow is generally opposite to the direction of flow of the semiconductor alloy gases through the decomposition region of the deposition chamber. However, the gaseous profiling material is now deposited at the downstream end of the glow discharge deposition chamber (relative to movement of the web of substrate material 11 therethrough) so that the gaseous species from the profiling gas manifold 52' are deposited adjacent the top surface of the depositing layer of semiconductor alloy material. More particularly, and viewing FIG. 4B, the semiconductor alloy material is shown deposited with a profiled or graded layer incorporated into the host matrix thereof by the solid line 56', which line is intended to indicate that the gaseous profiling material diffuses a given distance into the decomposition region with the least amount of gaseous profiling material being deposited into the semiconductor alloy layer at an upstream location (upstream relative to the movement of the web of substrate material 11 through the decomposition region).
As the web of substrate material 11 continues to move toward more downstream portions of the glow discharge deposition chamber, the amount of gaseous profiling material present in the plasma, as well as the amount of gaseous profiling material deposited onto the web of substrate material 11 continually increases so as to reach a maximum at the most downstream location (a location closest to the profiling gas introductory manifold 52'). The profiling or grading indicated by the phantom line 58' in FIG. 4B represents the deposition of a secondary gaseous precursor material from an elongated introductory manifold 54', generally similar in structure and function to introductory manifold 54 described with reference to FIG. 3A. The secondary gaseous precursor material is of the type that when subjected to the electromagnetic field developed between the web of substrate material 11 and the cathode plate 34 tends to decompose and deposit onto the web of substrate material 11 at a rate greater than the rate of deposition of the primary gaseous semiconductor alloy material introduced into the decomposition region from the manifold 36'. In this manner, the source of a secondary gaseous precursor material introduced from the introductory manifold 54' is depleted before that material can travel very far into the plasma region and hence, is graded or profiled into the depositing semiconductor alloy material. However, the secondary gaseous precursor material is graded into the bottom portion of the semiconductor alloy material (as compared to FIG. 3B in which it was graded into the upper portion thereof).
Whereas the aforementioned gas channeling and introducing systems of the glow discharge deposition chambers of FIGS. 3A and 4A were operative to deposit the aforedescribed profiles of dopant gases or secondary gaseous precursor materials, as indicated by the profiling curves of FIGS. 3B and 4B, said gas introductory systems were (1) limited to the deposition of only the precise profiles illustrated in FIGS. 3B and 4B, (2) unable to deposit the gaseous precursor species in any preselected and/or localized segments of the semiconductor alloy material, and (3) unable to deposit any preselected concentration of secondary gaseous precursor species within a given profile. For instance, it might be desirable to deposit, rather than a profiled layer of additional gaseous precursor material, a uniform layer of additional gaseous precursor material in the first deposited 1000 angstroms of the intrinsic semiconductor alloy material, or a graded layer thereof in the intermediate 1000 angstroms of intrinsic semiconductor alloy material, or a uniform layer thereof in the final 1000 angstroms of deposited intrinsic semiconductor alloy material. The prior gas introductory systems were simply not equipped to provide such alternatives of either graded or uniform disposition of a secondary gaseous precursor material at any desired location in the host matrix of the depositing semiconductor alloy material.
As a further, more specific example, it is desirable that in a tandem photovoltaic device, the more narrow band gap photovoltaic cell be formed by incorporating germanium into the host matrix of the silicon alloy material. However, it has been determined that germane gas, the gaseous precursor material from which the germanium is deposited, decomposes and is deposited onto the web of substrate material at a rate much greater than the rate of deposition of the silane gas, the gaseous precursor material from which silicon is deposited thereonto. Accordingly, the germane gas is depleted before reaching the end of the plasma region. Even by initially introducing a large rate of germane gas into the plasma region, more germane gas is incorporated into the host matrix of the silicon alloy adjacent the introductory manifold portion of the decomposition region than adjacent the exhaust region thereof. Therefore, it it were desired either to deposit a uniform layer of silicon-germanium alloy material onto the substrate, or to deposit a graded layer of silicon-germanium alloy material in the middle 1000 angstroms of deposited material, such prior gas channeling and introductory system would be incapable of accomplishing same. With the process gas introductory systems of FIGS. 3A and 4A, it would be impossible to avoid producing areas of the plasma region which were rich in the presence of germane gas while more downstream areas of the plasma region would be poor or depleted in the presence of said germane gas. Of course, the deposited semiconductor alloy material would correspondingly show upstream (relative to the germane gas introductory manifold) areas of rich germanium incorporation and downstream areas of depleted germanium incorporation in the host matrix thereof.
In addition to the aforementioned shortcomings of the gas channeling and introductory systems of FIGS. 3A and 4A, the most significant operational limitation of said prior art channeling and introductory systems resulted from the use of an elongated cathode which was specifically deployed in the chambers adapted for the deposition of the relatively thick layers of intrinsic semiconductor material. More specifically, since (1) the layer of intrinsic semiconductor alloy material may be as thick as about 3500 angstroms in a tandem photovoltaic cell, (2) the rate of deposition of said intrinsic semiconductor alloy material is relatively slow, (3) said intrinsic semiconductor alloy material is deposited continuously upon a moving web of substrate material, and (4) the doped layers of semiconductor alloy material may be less than about 100 angstroms thick, the cathode plates in the intrinsic deposition chamber past which the web of substrate material spacedly travels must be as much as about six feet in length. In order to insure that the amount of the gaseous precursor source of the primary semiconductor alloy material is not depleted at the downstream end of such an elongated plasma region, the velocity of the primary semiconductor alloy material introduced into the cathode region of the deposition chamber is substantially increased. Of course, since the rate of flow of the primary semiconductor alloy material is increased and the gaseous precursor dopant material can only be present in trace amounts in the host matrix of the intrinsic semiconductor alloy material, the flow rate of said gaseous pecursor dopant material introduced into the cathode region of the deposition chamber in a direction of travel parallel, but opposite to the direction of flow of the primary semiconductor alloy material, cannot be correspondingly increased so as to diffuse the preselected distance through the plasma region (against the oppositely directed flow of the primary semiconductor alloy material). The reason that the introductory flow rate of the gaseous precursor dopant material cannot be increased is that any such increase would cause too much of said dopant material to be deposited adjacent the manifold from which said dopant material is introduced. The consequential result was that the gradation of dopant profiling gas throughout at least a portion of the depositing layer of semiconductor alloy material could not heretofore be reliably controlled.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices substantially equivalent to those produced by their crystalline counterparts.
It is now possible to prepare amorphous silicon semiconductor alloys, by glow discharge or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. These techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent to Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; by vapor deposition as fully described in U.S. Pat. No. 4,217,374, issued to Stanford R. Ovshinsky and Masatsugu Izu, on Aug. 12, 1980, under the same title; and in U.S. Pat. No. 4,517,223 entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy by Stanford R. Ovshinsky, David D. Allred, Lee Walter and Stephen J. Hudgens. As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell while the short circuit current remains substantially constant.
Hamakawa et al, reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer there between. Each of the cells was made of an Si-H material of the same band gap as in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc increased and was proportional to the number of cells.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells mass production was limited to batch processing techniques by the inherent time consuming growth requirements of the crystals. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patents and applications: U.S. Pat. No. 4,400,409, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; U.S. patent application Ser. No. 244,386 filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,410,558 filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723 filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation system and Method; and U.S. Pat. No. 4,462,332 filed Mar. 19, 1982, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous semiconductor material, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor material, and the third chamber is dedicated for depositing an n-type amorphous semiconductor material.
It is for use with continuous deposition apparatus such as the systems described in the patent and patent applications cited hereinabove that the Cathode Assembly With Localized Profiling Capabilities of the present invention is directed. When equipped with the instant cathode assembly, said deposition apparatus is adapted to deposit uniform layers of semiconductor alloy material, said layers being precisely profilable in any specified manner regardless of the length of the cathode plates in any given deposition chamber.
These and further objects and advantages of the present invention will become clear from the drawings, the claims and the description of the preferred embodiment which follow.